As the features sizes of integrated circuits continue to decrease, the integrated circuit fabrication industry requires inspection tools with higher resolution, so as to resolve the smaller features of the circuits. As the term is used herein, “integrated circuit” includes devices such as those formed on monolithic semiconducting substrates, such as those formed of group IV materials like silicon or germanium, or group III-V compounds like gallium arsenide, or mixtures of such materials. The term includes all types of devices formed, such as memory and logic, and all designs of such devices, such as MOS and bipolar. The term also comprehends applications such as flat panel displays, solar cells, and charge coupled devices.
An improvement in the resolution of an inspection system can be achieved by using light sources that have shorter wavelengths than those currently employed. However, the choice of suitable sources, generally lasers, is very limited because the inspection system typically requires a laser with a high average power, but a low peak power. Additionally, performance stability—as well as costs of operation and maintenance—is of crucial importance.
Designing a laser that satisfies the above conditions becomes particularly challenging at wavelengths shorter than about two hundred nanometers. In general, there are two approaches to generating laser light below about two hundred nanometers, which is generally referred to herein as short wavelength light.
First, short wavelength light can be obtained directly from an excimer laser, such as an ArF excimer laser, which generally emits radiation with a wavelength of about 193 nanometers. Unfortunately, although these lasers operate at a desirably high average power, they also operate with relatively low repetition rates (up to about ten kilohertz), that tends to produce an extremely high peak power level, which is sufficient to damage most substrates and masks. In addition, industrial excimer lasers are usually very large and require extensive maintenance. Therefore, such lasers are of little use for most integrated circuit inspection systems.
Second, short wavelength light can be produced by a longer wavelength light source using a method called frequency conversion. This method involves one or more laser sources and a series of non-linear crystals, in which the conversion takes place. Currently available light sources, such as solid state lasers and fiber lasers, are capable of producing high-quality laser light in a wide range of output powers and repetition rates. However, the light that is generated by these sources is in the near-infrared range. Thus, multiple frequency conversion stages are required to produce short wavelength light.
Unfortunately, each of these multiple conversion stages reduces the output power of the laser, because the conversion efficiency of each stage tends to be significantly less than one. Furthermore, each frequency conversion stage tends to degrade the output beam quality, due to effects such as the spatial walk-off, and creates additional problems by increasing the number of elements of the system. For these reasons, it is vital to minimize the number of frequency conversion steps and to make every step as efficient as possible by using the most suitable crystals and specific wavelength combinations that produce the best results.
For example, several frequency-converting lasers that produce light at wavelengths below about two hundred nanometers have been demonstrated in the past. Most of them use more than one fundamental light source, such as two Nd:YAGs and Ti:Al2O3, two Nd:YLFs and Ti:Al2O3, Nd:YLF and OPO, and four or five frequency conversion stages. These devices are too complicated for use in an industrial inspection system.
The only system that uses a single fundamental infrared source includes a diode-pumped Er-doped fiber amplifier, whose 1,547 nanometer infrared output is frequency converted to produce an eighth harmonic at about 193 nanometers. The wavelength transformation is accomplished with five non-linear crystals. Among the drawbacks of this system are a low conversion efficiency and a poor beam quality that are caused by the frequency-converting crystals, as well as by thermal instabilities, due to the fact that in order to perform frequency conversion, some of the crystals have to be kept at high temperatures, above about 115° C.
What is needed, therefore, is a system that overcomes problems such as those described above, at least in part.